Blown multilaminate film is made by extruding a tubular multilaminate film from a die and inflating it with air supplied through the center of the die. U.S. Pat. No. 3,546,743 discloses a die for forming such film in which an annular stream of one resin is combined with an annular stream of another resin to form a composite annular stream which is expressed to form the film. Because a separate annular chamber is required for each resin, it is not practical to use such dies for making films having more than three layers.
West German OLS 2430231 discloses a die which has a single annular chamber but which can be used to make multilaminate film by feeding into the chamber a preformed stream of parallel layers of the resins used in making the film. As the stream enters the chamber, it divides into two substreams which flow in opposite directions through the chamber and then join together. The die has been in commercial use for more than one year, but it has been found that is is not practical to use the die to make film having more than about three layers because of the difficulty in (1) preforming a stream of more than three resin layers, (2) forming a satisfactory weld where the substreams join together in the annular chamber, and (3) matching the viscosities of more than three resins. (The viscosities of the resins must be reasonably close so that they all flow at about the same rate.)